1. Field of the Invention
The present invention broadly relates to measurement of visual sensitivity of a human subject and, more particularly, is concerned with a method of determining the human subject's visual contrast sensitivity function by objectively measuring threshold contrast levels in the steady state visual evoked potential waveform from the occipital cortex area of the subject's brain.
2. Description of the Prior Art
Over the past decade, a new method of testing vision has come into use in both the scientific and clinical communities. The method measures visual sensitivity, using targets called sine wave gratings, that are specified in terms of two variables: spatial frequency and contrast.
A sine wave grating pattern is a repeated sequence of light and dark bars that has a luminance profile, which varies sinusoidally about a mean luminance with distance. The width of one light and one dark bar of a grating pattern is one cycle, or the period of the grating pattern. The reciprocal of the period is the spatial frequency. Spatial frequency is expressed by the number of cycles of the grating pattern that occur over a particular distance, more commonly the cycles per unit of visual angle or per degree (cpd) which is dependent upon the viewing distance. The luminance difference of the light and dark bars determines the contrast of the grating pattern. The Michelson definition of contrast is most often used: EQU C=(L.sub.max -L.sub.min)/(L.sub.max +L.sub.min)
where L.sub.max and L.sub.min are the maximum and minimum luminances of the bars of the grating pattern. Examples of the sine wave grating patterns having low, medium, and high spatial frequencies at low and high contrasts are shown in FIG. 1 on page 7 of Air Force Aerospace Medical Research Laboratory report No. AFAMRL-TR-80-121, dated September 1981.
Psychophysical experiments have shown that sine wave grating patterns are an appropriate stimulus for analyzing visual function. The periodic or repeated luminance patterns can be varied in contrast and spatial frequency, as shown in the aforementioned report, to determine the visual contrast threshold. If the contrast of a grating pattern is increased from below its visibility to where the grating is just seen, then the pattern is said to have reached threshold contrast. The reciprocal of the threshold contrast is called contrast sensitivity. Grating patterns of different spatial frequencies require different amounts of contrast to reach threshold for a particular human subject. Psychophysical experiments have measured visual contrast thresholds for sine wave grating patterns from 0.25 cpd of visual angle to 25 cpd.
In a typical psychophysical experiment for measuring contrast sensitivity, the human subject views a video screen and adjusts the contrast of a sine wave grating pattern displayed on the screen until the bars are just at the subject's threshold of visibility. The measurements are repeated for a number of different bar widths (spatial frequencies). The reciprocal of contrast threshold is plotted as a function of spatial frequency to create a psychophysically-determined contrast sensitivity function (CSF). A typical contrast sensitivity function is shown in FIG. 2 on page 8 of the aforementioned report. A subject's CSF has been shown to directly relate to how well that individual detects and identifies targets covering a wide range in size.
While the above-described psychophysical technique for determining a subject's CFS is satisfactory for research purposes, it relies on the cooperation and understanding of the subject and hence may not be suitable for routine usage in a clinical setting. Consequently, various researchers have attempted to devise a more objective approach to determination of a subject's CSF.
Toward this goal, visual evoked potentials (VEPs) of human subjects have been studied for use in determining their contrast thresholds. Steady state VEPs are electrical responses of the brain to a flickering pattern, picked up by surface electrodes placed on the subject's scalp over the occipital cortex. The responses are synchronized in frequency to the fundamental, or some harmonic of the, frequency at which the stimulus is flickering. Part of the problem of recording VEPs is that they are buried in the noise produced by other electrical activity in the brain that is not related to visual function. Therefore, some type of filtering or signal averaging is usually required to extract the VEP signal from the noise. Considerable efforts have been expended in the past to develop the VEP into a clinical and research tool for assessing pattern vision.
Campbell and Maffei (see "Electrophysiological Evidence for the Existence of Orientation and Size Detectors in the Human Visual System," Journal of Physiology, 1970, vol. 207, pp. 635-652) studied the relationship between steady state visual evoked potentials (VEP) and threshold contrast sensitivity for flickering sine wave gratings. They measured VEP amplitude over a range of contrasts and showed that regressions fitted to plots of the logarithm of contrast versus the VEP amplitude intersected the contrast axis near the psychophysically measured threshold at each spatial frequency. The only difficulty with this method is the inordinate amount of time required to obtain a subject's CSF.
Harris, Atkinson and Braddick (see "Visual Contrast Sensitivity of a 6-Month-Old Infant Measured by the Evoked Potential," Nature, Dec. 9, 1976, vol. 264, pp. 570-571) used the Campbell and Maffei method to determine thresholds for contrast in a situation where direct psychophysical methods were not possible, that is, in human infants. Tyler, Apkarian, Levi and Nakayama (see "Rapid Assessment of Visual Function: An Electronic Sweep Technique for the Pattern Visual Evoked Potential," Invest. Ophthalmol. Vis. Sci., July 1979, vol. 18/7, pp. 703-713) developed an electronic spatial frequency sweep technique that assesses steady state VEPs more rapidly than the aforementioned threshold extrapolation method of Campbell and Maffei. However, Tyler et al admit that their technique does not produce contrast sensitivity functions and should only be used as an indicator of visual acuity.
While the above-mentioned approaches which utilize the VEPs of human subjects to arrive at an estimation of their CSFs are steps in the right direction, they still entail a considerable amount of time to be carried out, produce data of wide variability in individual responses, and require experienced personnel to perform the necessary regression analyses to arrive at the estimations of contrast thresholds. Therefore, a need exists for a more automated, faster approach to analyzing VEPs of human subjects and arriving at contrast threshold values of greater accuracy and repeatability.